Real come into the match on a high after all but securing a place in the Champions League semi-finals thanks to Tuesday's 3-0 win at APOEL Nicosia but have seen their advantage over second-placed Barca in La Liga whittled down to six points from 10 in recent weeks.
One of only two teams to beat Barca in the league this season, Osasuna are sixth, well placed to secure a berth in Europe next season, and their tough-tackling, well-organised side has only lost twice at home this term, to Racing Santander and Atletico Madrid during a poor run in January.
"They [Real] have three points at stake that could decide the league title, while we will simply go out there to win," Osasuna defender Miguel Flano told a news conference on Wednesday.
"It seems to me that Madrid are on good form, just like the rest of the season," he added. "They are a solid team, who allow themselves few errors and if they are given space they are extremely dangerous."
Victory for Jose Mourinho's side would stretch their lead over Barca to nine points, at least for a few hours until the Spanish, European and world champions host 11th-placed Athletic Bilbao in Saturday's late kick-off.
The season's surprise packages, Levante are fifth and a win at the Mestalla would put them level on points with their much wealthier neighbours and heap more pressure on Valencia coach Unai Emery, assuming he is still in charge by then.
Emery's side have their Europa League last-eight clash at AZ Alkmaar to negotiate on Thursday and with the board demanding improvement and fans increasingly restless, a poor performance in the Netherlands may enough to prompt a change of coach.
"We are ready to fight until our last drop of energy," forward Luis Garcia told a news conference on Tuesday.
